So new wheels and tires, yay! But I've noticed on the highway that I get steering wheel vibration that over the course of say 1-2 miles on I-75 that doesn't change, the vibration and shake will come and go.

I mean it'll be nothing, then I can feel (& visibily see) it fade in for awhile, then disappear, without any noticeable change in road condition... The seller did quite a bit of suspension work before I bought it, and I trust him entirely (Mike Helm, club races an MR2 and much experience). Trying to remember everything he said he did. Springs, struts, ...

Assuming the brand new tires are properly balanced, what should I check next? The coming and going is really what has me confused.

I wouldn't assume the new wheels and tires are properly balanced, personally. When dealing with the small light rolling stock on your car, it's going to be super sensitive to that. I've had this issue on my Miata three or four times over the past ten years when getting new tires.

Since I'm not sure what they are, I'd have to ask. I think I'm going to try the bushing above (Mike says he might have one laying around, big surprise) and maybe have them check the wheel balances. Just in case.